Semiconductor nanocrystal particle, method for preparing same, and device including same

ABSTRACT

A quantum dot including a core that includes a first semiconductor nanocrystal including zinc and selenium, and optionally sulfur and/or tellurium, and a shell that includes a second semiconductor nanocrystal including zinc, and at least one of sulfur or selenium is disclosed. The quantum dot has an average particle diameter of greater than or equal to about 13 nm, an emission peak wavelength in a range of about 440 nm to about 470 nm, and a full width at half maximum (FWHM) of an emission wavelength of less than about 25 nm. A method for preparing the quantum dot, a quantum dot-polymer composite including the quantum dot, and an electronic device including the quantum dot is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2019-0107637 filed in the Korean Intellectual Property Office on Aug.30, 2019, and all the benefits accruing therefrom under 35 U.S.C. § 119,the content of which in its entirety is incorporated herein byreference.

BACKGROUND 1. Field

A quantum dot, a method for preparing a quantum dot, and a deviceincluding a quantum dot are disclosed.

2. Description of the Related Art

Unlike bulk materials, intrinsic physical characteristics (e.g., energybandgaps, melting points, etc.) of nanoparticles may be varied orcontrolled by changing the size, of the nanoparticle. Semiconductornanocrystals also known as quantum dots is a semiconductor materialhaving a crystalline structure having a particle size of severalnanometers. The semiconductor nanocrystals have such a small size thatthe surface area per unit volume of the nanocrystal exhibits quantumconfinement effects, and thus may have different physicochemicalcharacteristics based on the semiconductor material itself. For example,a quantum dot if placed in an excited energy state, e.g., withirradiation from a light source or with electrical energy, e.g., anapplied electric current, may emit light in a wavelength correspondingto the size of the quantum dot. Accordingly, a quantum dot may absorblight from an excitation source and may emit light energy correspondingto its energy bandgap, that is, a quantum dot may exhibitelectroluminescent and photoluminescent properties. In the quantum dot,the energy bandgap may be adjusted by controlling the sizes and/or thecompositions of the semiconductor nanocrystal, which often can lead tophotoluminescence or electroluminescent characteristics of high colorpurity. Therefore, quantum dots may have various applications such as ina display device, an energy device, a bio-light emitting device, or thelike and are of interest.

A quantum dot having a core-shell structure is known to have slightlyincreased luminous efficiency due to surface passivation by a shell,however, may of the known quantum dots include cadmium, and cadmium isan element known to present serious environmental issues or concerns.Accordingly, of interest is the development of quantum dots that do notinclude cadmium, and yet still, exhibit excellent light emittingcharacteristics including high quantum efficiency.

SUMMARY

An embodiment provides an environmentally-friendly quantum dot capableof exhibiting improved light emitting characteristics, particularlyimproved blue light emitting characteristics.

Another embodiment provides the method for preparing a quantum dot.

Another embodiment provides an electronic device including the quantumdot.

A quantum dot according to an embodiment includes a quantum dotincluding a core including a first semiconductor nanocrystal includingzinc and selenium, and optionally sulfur and/or tellurium, and a shellincluding a second semiconductor nanocrystal including zinc and at leastone of sulfur or selenium, wherein the quantum dot has an averageparticle diameter of greater than or equal to about 13 nanometers (nm),an emission peak wavelength in a range of about 440 nm to about 470 nm,and a full width at half maximum (FWHM) of an emission wavelength ofless than about 25 nm, wherein the quantum dot is cadmium-free.

The quantum dot may have an average particle diameter of about 13 nm toabout 20 nm, and an emission peak wavelength in a range of about 445 nmto about 460 nm.

The quantum dot may exhibit quantum efficiency of greater than or equalto about 70%.

The first semiconductor nanocrystal may include zinc and selenium, or,or zinc, selenium, and tellurium.

The second semiconductor nanocrystal may include zinc and selenium, zincand sulfur, or zinc, selenium, and sulfur.

A ratio of a total moles of sulfur and selenium relative to moles ofzinc in the second semiconductor nanocrystal may be about 1 to about 2.

A method for preparing a quantum dot according to another embodimentincludes

providing a core including a first semiconductor nanocrystal includingzinc and selenium, and optionally sulfur and/or tellurium;

providing (i) a first solution including zinc-carboxylate, e.g.,(Zn(carboxylate)₂), a fluorine ion source, and a solvent, or (ii) asecond solution including ZnF₂ and a solvent;

adding the core to the first solution, or the second solution to providea first core solution or a second core solution, respectively,

adding at least one of a sulfur precursor, or a selenium precursor, tothe first core solution or the second core solution, and allowing thecore solutions to react,

wherein the core-shell quantum dot has an average particle diameter ofgreater than or equal to about 13 nm, an emission peak wavelength in arange of about 440 nm to about 470 nm, and a full width at half maximum(FWHM) of an emission wavelength of less than about 25 nm.

The fluorine ion source included in the first solution may include ZnF₂,HF, NH₄F, LiF, NaF, KF, BeF₂, MgF₂, CaF₂, SrF₂, CuF, AgF, AuF, ZnF₂,HgF₂, AlF₃, GaF₃, InF₃, SnF₂, PbF₂, or a combination thereof.

The second solution may further include a zinc precursor in addition tothe ZnF₂.

The zinc precursor of the second solution may include dimethyl zinc,diethyl zinc, zinc acetate, zinc acetylacetonate, zinc iodide, zincbromide, zinc chloride, zinc carbonate, zinc cyanide, zinc nitrate, zincoxide, zinc peroxide, zinc perchlorate, zinc sulfate, or a combinationthereof.

The zinc-carboxylate included in the first solution may includezinc-oleate (Zn(Oleate)₂), zinc palmitate, zinc stearate, zincoctanoate, or a combination thereof.

The zinc precursor in addition to ZnF₂ included in the second solutionmay include zinc-oleate (Zn(oleate)₂), zinc palmitate, zinc stearate,zinc octanoate, or a combination thereof.

A content of the zinc-carboxylate included in the first solution, or acontent of the ZnF₂ included in the second solution, may be less than orequal to about 20 mole percent zinc, based on a total moles of zinc inthe quantum dot.

Following the addition of the core to the second solution, a zincprecursor may be optionally added to the second core solution, andreact.

The core is added to the first solution or the second solution, andundergoes a first reaction, at least one of the sulfur precursor or theselenium precursor is then added to the first core solution or thesecond core solution and undergoes a second reaction.

The first solution or second solution may further include a ligandcompound.

The ligand compound may include RCOOH, RNH₂, R₂NH, R₃N, RSH, R₃PO, R₃P,ROH, RCOOR′, RPO(OH)₂, R₂POOH, or a combination thereof, wherein, R andR′ are independently a C1 to C24 aliphatic hydrocarbon group, or a C5 toC20 aromatic hydrocarbon group.

The adding of at least one of the sulfur precursor or the seleniumprecursor to the first or the second core solution includes a reactionat a temperature of about 300° C. to about 350 C.

The electronic device according to another embodiment may include thequantum dot according to an embodiment, or the quantum dot prepared bythe method according to an embodiment.

The quantum dot according to an embodiment is a cadmium-freeenvironmentally-friendly quantum dot that maintains a relatively largerparticle size and uniform shape, emits longer wavelength in the bluelight region of the spectrum, has a relatively smaller full width athalf maximum (FWHM), and a greater quantum efficiency. Thus, the quantumdot, or a plurality of quantum dots according to an embodiment, may beadvantageously used for a production of blue light emitting deviceshaving high luminance, high color reproducibility, and high reliability,and may also be used for various electronic devices such as variousdisplay devices including blue light emitting devices, biolabels (e.g.,for biosensors, bioimaging), photodetectors, solar cells, hybridcomposites, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a TEM image of the quantum dot according to Example 1.

FIG. 2 shows a TEM image of the quantum dot according to Example 2.

FIG. 3 shows a TEM image of the quantum dot according to Example 3.

FIG. 4 shows a TEM image of the quantum dot according to Example 4.

FIG. 5 shows a TEM image of the quantum dot according to Example 5.

FIG. 6 shows PL spectra of the quantum dots according to Examples 6 and7.

FIG. 7 shows UV-Vis spectra of the quantum dots according to Examples 6and 7.

FIG. 8 shows a TEM image of the quantum dot according to ComparativeExample 1.

FIG. 9 shows a TEM image of the quantum dot according to ComparativeExample 2.

FIG. 10 shows a TEM image of the quantum dot according to ComparativeExample 3.

FIG. 11 shows PL spectra of the quantum dots according to according toComparative Examples 4 and 5.

FIG. 12 shows UV-Vis spectra of the quantum dots according toComparative Examples 4 and 5.

FIG. 13 is an exploded view of an electronic device (LCD) including aquantum dot according to an embodiment.

DETAILED DESCRIPTION

Advantages and characteristics of this disclosure, and a method forachieving the same, will become evident referring to the followingembodiments together with the drawings attached hereto. However, theembodiments should not be construed as being limited to the embodimentsset forth herein.

If not defined otherwise, all terms (including technical and scientificterms) in the specification may be defined as commonly understood by oneskilled in the art, and therefore, should be interpreted as having ameaning that is consistent with their meaning in the context of therelevant art and the present disclosure, and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer, orsection. Thus, “a first element,” “component,” “region,” “layer,” or“section” discussed below could be termed a second element, component,region, layer, or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” “or” means “and/or.” As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

“About” as used herein is inclusive of the stated value and means withinan acceptable range of deviation for the particular value as determinedby one of ordinary skill in the art, considering the measurement inquestion and the error associated with measurement of the particularquantity (i.e., the limitations of the measurement system). For example,“about” can mean within one or more standard deviations, or within ±10%or ±5% of the stated value.

As used herein, the term “not including cadmium (or other harmful heavymetal)” refers to the case where a concentration of the cadmium (or theharmful heavy metal) may be less than or equal to about 100 ppm, lessthan or equal to about 50 ppm, less than or equal to about 10 ppm, oralmost zero. In an embodiment, substantially no amount of the cadmium(or other heavy metal) may be present or, if present, an amount of thecadmium (or other heavy metal) may be less than or equal to a detectionlimit or as an impurity level of a given analysis tool (e.g., aninductively coupled plasma atomic emission spectroscopy).

As used herein, “metal” refers to a metal or a semi-metal.

As used herein, “alkyl” refers to a linear or branched saturatedmonovalent hydrocarbon group (e.g., methyl, hexyl etc.). Unlessindicated otherwise, an “alkyl” may have any number of carbon atoms,e.g., from 1 to 60 carbon atoms, or 1 to 32 carbon atoms, or 1 to 24carbon atoms, or 1 to 12 carbon atoms.

As used herein, “alkenyl” refers to a linear or branched monovalenthydrocarbon group having at least one carbon-carbon double bond.

As used herein, “aryl” refers to a monovalent group formed by removingone hydrogen atom from at least one aromatic ring (e.g., phenyl ornaphthyl).

As used herein, “Group” refers to a group of Periodic Table.

As used herein, “Group II” refers to Group IIA and Group IIB, andexamples of Group II metal may be Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, “Group III” refers to Group IIIA and Group IIIB, andexamples of Group III metal may be Al, In, Ga, and TI, but are notlimited thereto.

As used herein, “Group IV” refers to Group IVA and Group IVB, andexamples of a Group IV metal may be Si, Ge, and Sn, but are not limitedthereto. As used herein, “metal” includes a semi-metal such as Si.

As used herein, “Group I” refers to Group IA and Group IB, and examplesmay include Li, Na, K, Rb, and Cs, but are not limited thereto.

As used herein, “quantum yield (QY)” is a value determined by thephotoluminescence spectrum obtained by dispersing quantum dots in atoluene solvent, based on the emission peak of ethanol solution of ananthracene dye (absorption (optical density) at 450 nm:0.1).

As nanocrystal particles including a semiconductor material, that is,quantum dots have photoluminescence characteristics with high colorpurity and are capable of controlling an energy bandgap depending upon asize and a composition, so they draw attentions as a material capable ofapplying to various fields, such as, for example, a display, energy,semiconductor, bio technologies, and the like.

Currently, quantum dots that demonstrate good performance in terms oflight emitting characteristics and stability will likely include cadmium(Cd). For example, quantum dots including Cd in the core and/or shellmay exhibit relatively high luminous efficiency. However, cadmium maycause severe environment/health problems and is a restricted element byRestriction of Hazardous Substances Directive (RoHS) in a plurality ofcountries. Therefore, it is desired to develop environmentally-friendlyquantum dots which may emit light of a desired wavelength while havingimproved light emitting characteristics (for example, as applied to anelectroluminescent device) and does not include cadmium.

Accordingly, research on cadmium-free semiconductor nanocrystals, thatis, semiconductor nanocrystals prepared by using a Group III-V compoundas the cadmium-free quantum dots, for example, quantum dots including acompound such as InP and the like in a core, quantum dots including aGroup II-VI compound, for example, a compound such as ZnSe and the likein a core is of great interest. Among known cadmium-free quantum dots,many have found that blue light-emitting quantum dots as applied to aphotoluminescence (PL) display, a electroluminescence (EL) display, orthe like are unsatisfactory in terms of performance due to lower quantumefficiency, lower photostability, and lower reliability than those ofgreen or red light emitting, cadmium-free quantum dots.

The present inventors have made an effort to prepare cadmium-freequantum dots that emit blue light, and exhibit improved quantumefficiency, and a lower full width at half maximum (FWHM) of an emissionwavelength. The cadmium-free quantum dots described herein have arelatively larger particle diameter, and tend to be of uniform shape andsize, and therefore, may be used in a light emitting device having highcolor reproducibility, high luminance, and high reliability.

A quantum dot according to an embodiment includes a quantum dotincluding a core including a first semiconductor nanocrystal includingzinc and selenium, and optionally sulfur and/or tellurium, and a shellincluding a second semiconductor nanocrystal including zinc and at leastone of sulfur or selenium, wherein the quantum dot has an averageparticle diameter of greater than or equal to about 13 nm, for example,about 13 nm to about 20 nm, an emission peak wavelength in a range ofabout 440 nm to about 470 nm, and a full width at half maximum (FWHM) ofemission wavelength of less than about 25 nm, wherein the quantum dot iscadmium-free.

The quantum dot according to an embodiment is a cadmium-free quantumdot, that is, a cadmium-free quantum dot including no cadmium in a coreand/or a shell, which has an emission peak wavelength in a range ofabout 440 nm to about 470 nm, and thus, emits blue light, and has a lowfull width at half maximum (FWHM) of less than about 25 nm, less than 20nm, or less than 16 nm. The quantum dots tend to have a relativelylarger particle diameter than that of other known blue light emittingquantum dots, and thus may be used to produce a blue light emittingdevice having high color reproducibility and high reliability. Inaddition, the quantum dot described herein has a quantum efficiency ofgreater than or equal to about 70%, for example, greater than or equalto about 75%, greater than or equal to about 80%, and thus may be usedto achieve a light emitting device having high luminance.

A shape of the quantum dot is not particularly limited. For example, thequantum dot may have a spherical shape, a pyramid shape, a multi-armed(or multi-pod) or cubic shape, but is not limited thereto. The quantumdot according to an embodiment, as shown in examples described later,primarily have a cubic shape, and in some instances a pyramidal shape,but is not limited to these two shapes. Rather, these two shapes tend tobe predominantly present in some embodiments. Moreover, a quantum dotaccording to an embodiment will tend to exhibit a shape and a size withgreater uniformity.

The first semiconductor nanocrystal included in the core of the quantumdot may include zinc and selenium, or zinc, selenium, and tellurium, orzinc, selenium, and sulfur. For example, the first semiconductornanocrystal may include zinc, selenium, and tellurium.

The second semiconductor nanocrystal included in the shell of thequantum dot may include zinc and selenium, zinc and sulfur, or zinc,selenium, and sulfur. For example, the second semiconductor nanocrystalmay include zinc, selenium, and sulfur. Moreover, in an embodiment, aratio of a total moles of sulfur and selenium relative to a moles ofzinc in the second semiconductor nanocrystal of the quantum dot may beabout 1 to about 2, for example, about 1 to about 1.5.

When the quantum dot has a ratio of a total moles of sulfur and seleniumrelative to moles of zinc in the second semiconductor nanocrystal withinthe range, a difference in the energies of bandgap between core andshell of the quantum dot increases, and accordingly, the quantum dot mayexhibit excellent light emitting characteristics of high quantumefficiency and a narrow full width at half maximum (FWHM).

As described above, the quantum dot according to an embodiment has acore/shell structure including a core including the first semiconductornanocrystal, and a shell including the second semiconductor nanocrystaldisposed on the surface of the core, for example, at least a portion ofthe surface or the whole surface of the core. The shell may be amulti-layered shell including at least two layers, wherein each layer,for example, neighboring layers of this multi-layered shell may have thesame or different compositions.

Without being bound by any particular theory, the shell of the quantumdot may play a role of effectively passivating the surface of the core,and thereby, increase luminous efficiency of the quantum dot and theoperational stability of the quantum dot. In addition, the shell mayplay a role of a physical barrier to secure stability of the core, whichcan be structurally or chemically sensitive depending on the surroundingenvironment. Accordingly, in an attempt to increase luminous efficiencyof the quantum dot as well as its operation stability, the quantum dotis prepared in a manner so that the shell may have a predeterminedthickness of the core/shell structure.

In general, however, as the shell of a core/shell quantum dot increasesin size, the shape of the quantum dot trends to non-uniformity, or afull width at half maximum (FWHM) of an emission wavelength tends toincrease. As a result, the quantum dot may not achieve or exhibit highcolor reproducibility in a light emitting device. Although a quantum dothaving a predetermined size or relatively larger particle size mayprovide an opportunity for blue light emission, particularly in the deepblue, i.e., 440 nm to 470 nm, 445 nm to 465 nm, 445 nm to 460 nm, 448 nmto 460 nm, 449 nm to 460 nm, 450 nm to 460 nm, or 450 nm to 455 nm, atechnical problem in achieving such a blue light emitting quantum dotcan lie in controlling the shape or uniformity of the quantum dot, whichis a difficult technical task with no easy solution. Accordingly, a bluelight emitting quantum dot that emits blue light, has a high luminousefficiency, a narrow full width at half maximum (FWHM), and has a largeparticle size, and a relative uniform particle shape is difficult toprepare.

Conventionally, chlorine is included in a shell during the preparationof a quantum dot having a core/shell structure to increase luminousefficiency, achieve blue light emission, and increase shell thickness,however, the drawback to the presence of chlorine will likely result ina distortion (or non-uniform) in the shape of the particle and anincrease in the full width at half maximum (FWHM) of an emissionspectrum.

The quantum dot according to an embodiment, which emits blue light inthe region 440 nm to 470 nm, 445 nm to 465 nm, 445 nm to 460 nm, 448 nmto 460 nm, 449 nm to 460 nm, 450 nm to 460 nm, or 450 nm to 455 nm, andhas high luminous efficiency and simultaneously, a narrow full width athalf maximum (FWHM) of an emission spectrum may be prepared by a methodincluding adding a core including the first semiconductor nanocrystal toa solvent including a zinc-carboxylate (Zn(carboxylate)₂) and a fluorineion source, or a zinc fluoride (ZnF₂) and solvent, adding precursors ofthe second semiconductor nanocrystal thereby forming the shell aroundthe core (or the first semiconductor nanocrystal). The quantum dot thatresults from such a process has a relatively large particle size with auniform particle shape.

A method for preparing a core-shell quantum dot according to embodimentincludes

providing a core including a first semiconductor nanocrystal includingzinc and selenium, and optionally sulfur and/or tellurium,

preparing (i) a first solution including zinc-carboxylate(Zn(carboxylate)₂), a fluorine ion source, and a solvent, or (ii) asecond solution including ZnF₂ and a solvent,

adding the core to the first solution or the second solution to providea first core solution or a second core solution, respectively; and

adding at least one of a sulfur precursor or a selenium precursor to thefirst core solution or the second core solution, and allowing the coresolutions to react.

As noted above, the technology of increasing the shell thickness byincluding chlorine in the shell may increase luminous efficiency asdisclosed (Korean Patent Laid-Open Publication No. 2017-0074858A1published in Dec. 22, 2015.). The Laid-Open patent discloses the effectsaccomplished by introducing a halogen element such as chlorine to theshell and in the working Examples, demonstrates a luminous efficiencydifference depending on whether or not chlorine is present, however,there does not appear to be a discussion regarding the formation of afirst core solution or a second core solution as described above, andthen adding precursors to form a core/shell quantum dot.

In addition, the Laid-Open patent specifically discloses Examples of thecore including an InP compound, but not necessarily a firstsemiconductor nanocrystal including zinc and selenium, and optionally,sulfur and/or tellurium. Furthermore, full widths at half maximum (FWHM)of emission spectra of the quantum dots of the Laid-Open patent areabout 40 nm, which is much larger than about 25 nm or 20 nm, and have asignificantly greater than a full width at half maximum (FWHM) of anemission spectrum than the quantum dot according to an embodiment.

Accordingly, the method of preparing a quantum dot having a core/shellstructure includes adding the core including the first semiconductornanocrystal including zinc and selenium, and optionally, sulfur and/ortellurium to (i) a first solution including the zinc-carboxylate (Zn(carboxylate)₂), a fluorine ion source, and a solvent, or (ii) a secondsolution including the ZnF₂ and a solvent, and then adding theprecursors of a second semiconductor nanocrystal to form a shell aroundthe core. A quantum dot prepared by the method according to anembodiment are different from the quantum dot prepared by a methodincluding adding chlorine or halogen to a shell and a correspondingmethod for preparing the quantum dot.

Without wishing to be bound by a specific theory, in the method ofpreparing the quantum dot according to an embodiment, when the zincfluoride (ZnF₂) is used for the second solution, or a zinc-carboxylateand a fluorine ion source are used for the first solution, as precursorsof the zinc fluoride (ZnF₂), to form the shell around the core, the zincof the zinc fluoride (ZnF₂) is believed to play a role as a precursor ofzinc of the second semiconductor nanocrystal and to form the shell withother precursors of the second semiconductor nanocrystal to increase athickness of the shell. Moreover, the fluorine ion may play a role incontrolling the shell growth to provide uniform growth of the shell.Accordingly, according to the method of preparing the quantum dot thatincludes adding the core to the second solution including the zincfluoride or the first solution including zinc-carboxylate, which is aprecursor of the zinc fluoride in the presence of a fluorine ion source,and then, adding the precursor of the second semiconductor nanocrystalshell to provide a second or first core solution, respectively, thequantum dot may have an increased size of greater than or equal to about13 nm. The result of which is a quantum dot that can emit blue light,e.g., with a wavelength of about 440 nm to 470 nm, and improved luminousefficiency. Moreover, the quantum dot particle can maintain a uniformsize and shape, and a full width at half maximum (FWHM) of an emissionwavelength of less than about 25 nanometers.

However, as aforementioned, the prior art including halide in a shell byadding metal halide (e.g., ZnCl₂, etc.) exhibits an effect of increasinga shell thickness and thus luminous efficiency, but, as the shellincreases in thickness, the chloride appears to play little or no rolein controlling the uniformity of the quantum dot particle size or shape.The Laid-Open patent introduces the metal halide as a second halogensource and discloses Example of introducing a fluorine ion source suchas HF (hydrogen fluoride) and the like as a first halogen source, butfails to disclose a solution including the fluorine ion source alongwith zinc-carboxylate, for example, zinc-oleate and the like. Moreover,the Laid-Open patent appears not to provide a method of adding the coreto the solution to prepare the semiconductor nanocrystal having acore-shell structure, as according to the embodiments described herein.

In an embodiment, a content of the zinc-carboxylate (Zn(carboxylate)₂)included in the first solution, or a content of the zinc-fluoride (ZnF₂)included in the second solution may account for less than or equal toabout 20 mole percent (mol %), for example, about 1 mol % to about 20mol %, about 5 mol % to about 20 mol %, about 5 mol % to about 15 mol %,or about 10 mol %, based on total moles of zinc included in the core andshell of the quantum dot, but are not limited thereto.

The fluorine ion source included in the first solution may include asalt compound of a metal or a non-metal and a fluorine ion, for example,ZnF₂, HF, NH₄F, LiF, NaF, KF, BeF₂, MgF₂, CaF₂, SrF₂, CuF, AgF, AuF,HgF₂, AlF₃, GaF₃, InF₃, SnF₂, PbF₂, or a combination thereof, but is notlimited thereto. In an embodiment, the fluorine ion source included inthe first solution may include ZnF₂, HF, or a combination thereof.

For example, the zinc-carboxylate included in the first solution mayinclude zinc-oleate (Zn(oleate)₂), zinc palmitate, zinc stearate, orzinc octanoate. The zinc-carboxylate may also be used as a zincprecursor to form a second semiconductor nanocrystal included in theshell. However, this zinc-carboxylate is not included alone, and asdescribed above, it may be used with the fluorine ion source to preparethe quantum dot according to an embodiment.

Meanwhile, the second solution may include ZnF₂ alone or may includeanother zinc precursor in addition to the ZnF₂. Any zinc precursor canbe used in addition to ZnF₂, as a precursor of zinc to form the secondsemiconductor nanocrystal, and is not limited to a specific compound.For example, the zinc precursor may include dimethyl zinc, diethyl zinc,zinc acetate, zinc acetylacetonate, zinc iodide, zinc bromide, zincchloride, zinc carbonate, zinc cyanide, zinc nitrate, zinc oxide, zincperoxide, zinc perchlorate, zinc sulfate, or a combination thereof, butis not limited thereto.

The first solution or second solution may be heated to a desiredtemperature. The heating of the first solution as desired may includeheating the first solution at a temperature of greater than or equal toabout 40° C., for example, greater than or equal to about 50° C.,greater than or equal to about 60° C., greater than or equal to about70° C., greater than or equal to about 80° C., greater than or equal toabout 90° C., greater than or equal to about 100° C., or greater than orequal to about 110° C. under a vacuum or inert gas atmosphere. Inaddition, it may include heating the first solution at a temperature ofgreater than or equal to about 100° C., for example, greater than orequal to about 150° C., or greater than or equal to about 170° C. undera nitrogen atmosphere. In an embodiment, after adding the core includingthe first semiconductor nanocrystal to the first solution or the secondsolution to provide a first core solution or a second core solution,respectively, and before adding the shell-forming precursors includingthe second semiconductor nanocrystal to the first or second coresolutions, the first core solution or the second core solution may beheated to a desired reaction temperature. At least one precursor of thesulfur precursor or selenium precursor is then added to the heated coresolutions, and a reaction between the core and the shell-formingprecursors results, thereby forming the shell around the core, whichincludes the first semiconductor nanocrystal. The reaction temperatureis not particularly limited and may be appropriately selected inconsideration of the types of the precursors used, the zinc-carbolate,the fluorine ion source, zinc-fluoride, and the solvent used, and thelike. For example, the reaction temperature may be greater than or equalto about 200° C., for example, greater than or equal to about 220° C.,greater than or equal to about 250° C., greater than or equal to about270° C., greater than or equal to about 280° C., greater than or equalto about 290° C., greater than or equal to about 300° C., greater thanor equal to about 310° C., greater than or equal to about 320° C.,greater than or equal to about 340° C., or greater than or equal toabout or 350° C. For example, the reaction temperature may be less thanor equal to about 360° C., for example, less than or equal to about 50°C., less than or equal to about 340° C., less than or equal to about330° C., or less than or equal to about 320° C. In an embodiment, thereaction temperature may be in a range of about 300° C. to about 350° C.

As described above, when the second solution includes ZnF₂ alone but nozinc precursor, a reaction temperature forming a shell on the surface ofa semiconductor nanocrystal core may be higher than that of a solutionthat includes a zinc precursor. For example, when the core is added tothe first solution or the second solution, a reaction of forming a shellby adding at least one of the sulfur precursor or the selenium precursormay be performed at about 300° C. to about 350° C. Herein, when thesecond solution further includes additional zinc precursors, forexample, zinc-oleate, ZnCl₂, or a combination thereof along with ZnF₂,the reaction temperature for forming the shell may be within a range ofabout 300° C. to about 340° C., for example, about 320° C. to about 340°C. In another embodiment, when the second solution includes ZnF₂ alone,i.e., without additional zinc precursors, the reaction temperature forforming the shell may be about 340° C. to about 350° C., for example,about 350° C.

The reaction time is not particularly limited, but appropriatelyselected. For example, the reaction may be permitted to take place forgreater than or equal to about 5 minutes, greater than or equal to about10 minutes, or greater than or equal to about 15 minutes but is notlimited thereto. When the precursor mixture is injected by stages, apredetermined time in each step may be, for example, greater than orequal to about 5 minutes, greater than or equal to about 10 minutes, orgreater than or equal to about 15 minutes. The reaction may be performedunder an inert gas atmosphere or in the air or under vacuum, but is notlimited thereto.

The method for preparing a quantum dot according to an embodiment may besimilar to a general method of preparing a quantum dot except that afterpreparing the first solution or the second solution, a core is added toprovide a first core solution or a second core solution, respectively.Subsequently at least one of additional precursors, that is, a sulfurprecursor or a selenium precursor for forming the shell is added to thecore solutions. For example, after adding the core including the firstsemiconductor nanocrystal to the first solution or the second solution,the addition of the precursor of the second semiconductor nanocrystalforming the shell may be performed by dividing an entire amount of theprecursors of the added second semiconductor nanocrystal into two ormore portions and adding portion by portion at predetermined timeintervals. Alternatively, when two or more precursors of the secondsemiconductor nanocrystal are present, each of the two or moreprecursors may be divided into predetermined portions, and alternativelyadded at select time intervals. This addition may be appropriatelyadjusted by a person having an ordinary skill in the related art inorder to appropriately adjust desired characteristics in each quantumdot. Accordingly, in an embodiment, after adding the core to the firstsolution or the second solution, at least one of the sulfur precursor orthe selenium precursor may be added to the core solutions at once ordivided into several portions and added over several time intervals. Atleast one of the sulfur precursor or the selenium precursor may includeone or two or more types of precursors, respectively. When at least oneof the sulfur precursor or selenium precursor includes two or more typesof the first precursor and the second precursor, each precursor may beadded simultaneously or at a predetermined time intervals. Eachprecursor may be added to the reaction solution at the same or differenttemperature. The first precursor is mixed with the same/different typesof ligand and/or solvent in consideration of a shell composition of afinal quantum dot, and thus, used as a first mixture, and the secondprecursor may be mixed with the same/different types of ligand and/orsolvent in consideration of the shell composition of the final quantumdot, and then, additionally added to the first mixture at least once,for example, twice, three times, four times, five times, or more times.

The type of the sulfur-containing precursor is not particularly limited.The sulfur-containing precursor may include a sulfur powder, hexanethiol, octane thiol, decane thiol, dodecane thiol, hexadecane thiol,mercapto propyl silane, sulfur-trioctylphosphine (S-TOP),sulfur-tributylphosphine (S-TBP), sulfur-triphenylphosphine (S-TPP),sulfur-trioctylamine (S-TOA), bis(trimethylsilyl) sulfide, ammoniumsulfide, sodium sulfide, or a combination thereof.

The type of the selenium-containing precursor is not particularlylimited. For example, the selenium-containing precursor may includeselenium, selenium-trioctylphosphine (Se-TOP),selenium-tributylphosphine (Se-TBP), selenium-triphenylphosphine(Se-TPP), or a combination thereof, but is not limited thereto.

The solvent may be selected from C6 to C22 primary amine such ashexadecylamine; C6 to C22 secondary amine such as dioctylamine; C6 toC40 tertiary amine such as trioctylamine; a nitrogen-containingheterocyclic compound such as pyridine; C6 to C40 aliphatic hydrocarbon(e.g., alkane, alkene, alkyne, etc.) such as hexadecane, octadecane,octadecene, or squalane; C6 to C30 aromatic hydrocarbon such asphenyldodecane, phenyltetradecane, or phenyl hexadecane; phosphinesubstituted with a C6 to C22 alkyl group such as trioctylphosphine;phosphine oxide substituted with a C6 to C22 alkyl group such astrioctylphosphine oxide; C12 to C22 aromatic ether such as phenyl etheror benzyl ether, or a combination thereof. The type of solvent may beappropriately selected in consideration of the types of precursors,zinc-carboxylate, fluorine ion source, and organic ligand.

The quantum dot according to an embodiment may include organic ligandscoordinated on its surface. The organic ligand may use generally knownligand compounds but is not particularly limited. For example, theligand compound may include RCOOH, RNH₂, R₂NH, R₃N, RSH, RH₂PO, R₂HPO,R₃PO, RH₂P, R₂HP, R₃P, ROH, RCOOR′, RPO(OH)₂, R₂POOH (wherein, R and R′are independently a C1 to C24 alkyl group, a C2 to C24 alkenyl group, aC2 to C24 alkynyl group, or a C6 to C20 aryl group), or a combinationthereof. The organic ligand compound may coordinate the surface of theobtained nanocrystal, may help the nanocrystal be well dispersed in thesolution, and may also have an effect on light emitting and electricalcharacteristics of the quantum dot. Specific examples of the organicligand compound may include methane thiol, ethane thiol, propane thiol,butane thiol, pentane thiol, hexane thiol, octane thiol, dodecane thiol,hexadecane thiol, octadecane thiol, benzyl thiol; methane amine, ethaneamine, propane amine, butyl amine, pentyl amine, hexyl amine, octylamine, dodecyl amine, hexadecyl amine, octadecyl amine, dimethyl amine,diethyl amine, dipropyl amine; methanoic acid, ethanoic acid, propanoicacid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid,octanoic acid, dodecanoic acid, hexadecanoic acid, octadecanoic acid,oleic acid, benzoic acid; phosphine such as substituted or unsubstitutedmethyl phosphine (e.g., trimethyl phosphine, methyldiphenyl phosphine,etc.), substituted or unsubstituted ethyl phosphine (e.g., triethylphosphine, ethyldiphenyl phosphine, etc.), substituted or unsubstitutedpropyl phosphine, substituted or unsubstituted butyl phosphine,substituted or unsubstituted pentyl phosphine, substituted orunsubstituted octylphosphine (e.g., trioctylphosphine (TOP)), and thelike; phosphine oxide such as substituted or unsubstituted methylphosphine oxide (e.g., trimethyl phosphine oxide, methyldiphenylphosphine oxide, etc.), substituted or unsubstituted ethyl phosphineoxide (e.g., triethyl phosphine oxide, ethyldiphenyl phosphine oxide,etc.), substituted or unsubstituted propyl phosphine oxide, substitutedor unsubstituted butyl phosphine oxide, substituted or unsubstitutedoctylphosphine (e.g., trioctylphosphine oxide (TOPO)), and the like; adiphenyl phosphine or triphenyl phosphine compound, or an oxide compoundthereof; phosphonic acid, and the like, but are not limited thereto. Theorganic ligand compound may be used alone or as a mixture of two ormore.

In order to make a quantum dot that includes an organic ligand on asurface thereof according to an embodiment, the first solution or thesecond solution may further include a ligand compound. This ligandcompound may be at least one of various ligand compounds exemplifiedabove, but is not limited thereto.

In the first solution, each amount of the zinc-carboxylate(Zn(carboxylate)₂), the fluorine ion source, and the solvent may be, forexample, appropriately selected in consideration of a desired shellthickness, types of precursors, and the like but is not particularlylimited thereto. In the second solution, each amount of ZnF₂ and thesolvent, and also the zinc precursor, if is additionally included, maybe appropriately selected in consideration of a shell thickness, typesof precursors, and the like as desired, but is not particularly limitedthereto.

On the surface of the core including the first semiconductornanocrystal, for example, a portion or all of the surface, it ispossible to prepare a shell including the second semiconductornanocrystal, and thus to provide a quantum dot having a core/shellstructure through the reactions among the zinc-carboxylate,zinc-fluoride, sulfur precursor, selenium precursor, or a combinationthereof.

The method for preparing the quantum dot may further include adding anonsolvent to the reaction product between the core and theshell-forming precursors to separate the nanocrystal. The nonsolvent maybe a polar solvent that is miscible with the solvent used in thereaction but cannot disperse the nanocrystals therein. The nonsolventmay be selected depending the solvent used in the reaction and may befor example, acetone, ethanol, butanol, isopropanol, ethanediol, water,tetrahydrofuran (THF), dimethylsulfoxide (DMSO), diethyl ether,formaldehyde, acetaldehyde, a solvent having a similar solubilityparameter to the foregoing solvents, or a combination thereof.

The separation of the prepared quantum dot may be performed through acentrifugation, precipitation, chromatography, or distillation. Theseparated nanocrystal may be added to a washing solvent and washed, ifnecessary. The separated nanocrystal may be a nanocrystal on whichligand compounds are coordinated. The washing solvent has no particularlimit and may have a similar solubility parameter to that of the ligand,and may, for example, include hexane, heptane, octane, chloroform,toluene, benzene, and the like.

In another embodiment, a quantum dot polymer composite includes apolymer matrix; and the quantum dot is dispersed in the polymer matrix.

The polymer matrix may be a thiol-ene polymer, a (meth)acrylate-basedpolymer, a urethane-based resin, an epoxy-based polymer, a vinyl-basedpolymer, a silicone resin, or a combination thereof. The thiol-enepolymer may be one disclosed in US-2012-0001217-A1, the content of whichis incorporated herein by reference in its entirety. The(meth)acrylate-based polymer, the urethane-based resin, the epoxy-basedpolymer, the vinyl-based polymer, and the silicone resin may besynthesized by known methods, or monomers or polymers thereof may becommercially available.

A content of the quantum dot in the polymer matrix may be appropriatelyselected and is not particularly limited. For example, the content ofthe quantum dot in the polymer matrix may be greater than or equal toabout 0.1 weight percent (wt %) and less than or equal to about 30 wt %based on a total weight of the composite, but is not limited.

A method of preparing the quantum dot polymer composite may include themixing of a dispersion component including the quantum dot with asolution including a polymer, and then removing a solvent therefrom, butis not limited thereto. Alternatively, the quantum dot polymer compositemay be obtained by dispersing the quantum dot in a monomer mixture forforming the polymer, and polymerizing the mixture to form the composite.Such a quantum dot-polymer composite may be a quantum dot sheet (QDsheet).

Another embodiment provides an electronic device including theaforementioned quantum dot. Details of the semiconductor nanocrystalparticle are the same as described above. The device may be a lightemitting diode (LED), an organic light emitting diode (OLED), variousdisplays (e.g., liquid crystal display (LCD)), a sensor, a solar cell,or an imaging sensor, but is not limited thereto. FIG. 3 shows asimplified stacking structure of a liquid crystal display (LCD)including the quantum dot sheet among these devices. The generalstructure of the liquid crystal display (LCD) is well known, and FIG. 3shows the simplified structure.

Referring to FIG. 13 , the liquid crystal display may have a structurethat includes a reflector, a light guide (LGP) and a blue LED lightsource (Blue-LED), a quantum dot-polymer composite sheet (QD sheet),various optical films (e.g., a prism, and a double brightness enhancefilm (DBEF) are stacked, and a liquid crystal panel is disposed thereon.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, they are exemplary embodiments of thepresent invention, and the present invention is not limited thereto.

EXAMPLES

Analysis Methods

[1] Photoluminescence Analysis

Photoluminescence (PL) spectra of the prepared nanocrystals are obtainedusing a Hitachi F-7000 spectrometer at an irradiation wavelength of 372nm.

[2] UV Spectroscopy

A Hitachi U-3310 spectrometer is used to perform a UV spectroscopy andobtain UV-Visible absorption spectra.

[3] TEM Analysis

(1) Transmission electron microscope photographs of the preparednanocrystals are obtained using an UT F30 Tecnai electron microscope.

(2) TEM-EDX analysis (element mapping) is performed using UT F30 Tecnaielectron microscope.

[4] ICP Analysis

An inductively coupled plasma atomic emission spectroscopy (ICP-AES) isperformed using Shimadzu ICPS-8100.

[5] HRTEM Analysis

HRTEM analysis is performed using TEM-Titan G2.

[6] X ray Diffraction Analysis

Crystal structures of the semiconductor nanocrystals are confirmed byperforming XRD analysis using a Philips XPert PRO apparatus at a powerof 3 kW.

Syntheses are performed under an inert gas atmosphere (under a nitrogenflowing condition), unless particularly stated otherwise.

Synthesis Example 1: Preparation I of ZnTeSe Core

Selenium and tellurium are dispersed in trioctylphosphine (TOP) toobtain a 2 M Se/TOP stock solution and a 0.1 M Te/TOP stock solution,respectively.

0.125 millimoles (mmole) of zinc acetate, 0.25 mmole of palmitic acid,and 0.25 mmole of hexadecylamine are added to a reactor containing 10 mLof trioctylamine, and the reaction mixture is heated to 120° C. undervacuum. After one hour, the atmosphere in the reactor is filled withnitrogen to provide an inert atmosphere.

After heating the reactor at 300° C., the Se/TOP stock solution and theTe/TOP stock solution are rapidly injected into the reactor in a Te/Semole ratio of 1/25. After 10 minutes, 30 minutes, or 60 minutes, thereaction solution is rapidly cooled down to room temperature, acetone isadded to the reaction mixture, and the obtained mixture is centrifugedto obtain a precipitate, and the precipitate is dispersed in toluene.

The obtained (first) semiconductor nanocrystal has a first absorptionmaximum wavelength ranging from 400 nm to 430 nm and a maximum peakemission wavelength ranging from 430 nm to 460 nm. The semiconductornanocrystal has quantum efficiency ranging from about 30% to 40%.

Synthesis Examples 2-1 to 2-5: Preparation II of ZnTeSe Core

A core is prepared according to the same method as Synthesis Example 1(reaction time: 30 to 60 minutes) except that the mole ratio betweenselenium and tellurium is adjusted as shown in Table 1.

Each maximum emission peak wavelength and full width at half maximum(FWHM) of the prepared cores, and a tellurium weight ratio in theprepared (first) semiconductor nanocrystals (examined by ICP) are shownin Table 1.

TABLE 1 Maximum Te/Se mole emission ratio of peak reaction wavelengthFWHM Te content system (nm) (nm) (wt %) Syn. Example 2-1 0 422 24 0 Syn.Example 2-2 1/50 431 48 1.78 Syn. Example 2-3 1/30 441 57 2.82 Syn.Example 2-4 1/25 445 57 3.34 Syn. Example 2-5 1/8  478 67 7.5

Example 1: Preparation of ZnTeSe Core/ZnSe/ZnS Shell Quantum Dot

At room temperature, 0.5 mole of ZnF₂ is added to 100 mL of squalane ina flask, 0.6 mole of zinc acetate and 0.09 mole of ZnCl₂ as a zincprecursor are also added to the flask, and the mixture is vacuum-treatedat 120° C. for 10 minutes. The inside of the flask is filled withnitrogen (N₂) and is heated to 180° C. Subsequently, the ZnTeSe coreprepared in Synthesis Example 1 (reaction time: 60 minutes) is added tothe flask within 10 seconds, 0.75 mmole of zinc acetate is subsequentlyslowly added thereto, and the reaction temperature is raised to 340° C.and reacted for 10 minutes. Then, 0.75 mmole of zinc acetate and 0.5mmole of Se/TOP are slowly injected into the flask, and then reacted for10 minutes. Subsequently, 0.75 mmole of zinc acetate, 0.5 mmole ofSe/TOP, and 1.0 mmole of S-TOP are added, and reacted for 10 minutes.0.8 mmole of zinc acetate, 0.5 mmole of Se/TOP, and 1.5 mmole of S/TOPare added, and reacted for 10 minutes. Lastly, a mixed solution of 0.8mmole of zinc acetate, 0.5 mmole of Se/TOP, and 1.24 mmole of S/TOP isslowly added, and reacted for 20 minutes.

After the reaction is completed, the reactor is cooled, and the preparednanocrystals are centrifuged with ethanol and dispersed in toluene.

UV-Vis spectroscopy, a photoluminescence analysis, and a transmissionelectron microscopic analysis of the prepared nanocrystals (core/shellquantum dots) are performed, and the results are shown in FIG. 1 andTable 2.

Referring to the photoluminescence analysis result, the quantum dotsexhibit a maximum emission peak of 452 nm (a full width at half maximum(FWHM): 16 nm) and quantum efficiency of 85%. In other words, thequantum dots emit blue light and exhibit excellent light emittingcharacteristics of a narrow full width at half maximum (FWHM) and highquantum efficiency.

In addition, referring to FIG. 1 , the quantum dots tend to haveprimarily a cubic shape with some quantum dots having a pyramidal shape.The quantum dots have a particle diameter within a range of about 15.9nm±1.2 nm (8%). Inductively coupled plasma atomic emission spectroscopyof the ZnTeSe core and the ZnTeSe (core)/ZnSeS/ZnS (shell) is performed,and the results are listed as a mole ratio relative to Zn in Table 2.

Example 2: Preparation of ZnTeSe Core/ZnSe/ZnS Shell Quantum Dot

A quantum dot of ZnTeSe (core)/ZnSeS/ZnS (shell) is prepared accordingto the same method as Example 1 except that ZnCl₂ is not added to thereaction solution.

UV-Vis spectroscopy, a photoluminescence analysis, and a transmissionelectron microscopic analysis of the prepared nanocrystals (core/shell)are performed, and the results are shown in FIG. 2 and Table 2.

Referring to the photoluminescence analysis result, the quantum dotsexhibit a maximum emission peak of 451 nm (a full width at half maximum(FWHM): 15 nm) and quantum efficiency of 70%. In other words, thequantum dots emit blue light and exhibit excellent light emittingcharacteristics of high quantum efficiency and a narrow full width athalf maximum (FWHM).

In addition, referring to the TEM image of FIG. 2 , about half of thesemiconductor nanocrystal particles has a pyramidal shape, and the otherhalf a cubic shape. In addition, the quantum dots may have a particlediameter ranging from about 14.2 nm±1.1 nm (8%).

Inductively coupled plasma atomic emission spectroscopy of the ZnTeSecore and the ZnTeSe (core)/ZnSeS/ZnS (shell) is performed, and theresults are listed as a mole ratio relative to Zn in Table 2.

TABLE 2 A. QY (H Co., (Se + S)/ core PL (@372) Ltd.) Zn Se S Zn S/Se Ex.1 Syn. 452 16 85 4.45 2.0 3.74 1.29 1.87 Ex. 1 Ex. 2 451 15 70 4.45 2.03.74 1.29 1.87

Example 3: Preparation of ZnTeSe Core/ZnSe/ZnS Shell Quantum Dot

1.8 mmole of zinc oleate, 0.5 mole of ZnF₂, and 0.09 mole of ZnCl₂ areadded to 100 mL of squalane in a flask at room temperature, and then,vacuum-treated at 120° C. for 10 minutes. The inside of the flask isfilled with nitrogen (N₂), and the reaction temperature is raised to180° C. Subsequently, the ZnTeSe core prepared in Synthesis Example 1(reaction time: 60 minutes) is added to the flask within 10 seconds, 0.5mmole of Se/TOP is subsequently slowly added to the flask, and thereaction temperature raised to 280° C. Then, 0.7 mmole of S/TOP isadded, and the reaction temperature raised to 320° C. and reacted for 10minutes. Subsequently, a mixed solution of 0.5 mmole of Se/TOP and 0.9mmole of S/TOP is slowly added, and reacted for 20 minutes. Lastly, amixed solution of 0.5 mmole of Se/TOP and 1.2 mmole of S/TOP is slowlyadded, and reacted for 20 minutes.

After the reaction is completed, the reactor is cooled, and the preparednanocrystals are centrifuged with ethanol and dispersed in toluene.

UV-Vis spectroscopy, a photoluminescence analysis, and a transmissionelectron microscopic analysis of the prepared nanocrystals (core/shell)are performed, and the results are shown in FIG. 3 and Table 3.

Referring to the photoluminescence analysis result, the quantum dotsexhibit a maximum emission peak of 451 nm (a full width at half maximum(FWHM) of 21 nm) and quantum efficiency of 82%. In other words, thequantum dots emit blue light and exhibit excellent light emittingcharacteristics of high quantum efficiency and a narrow full width athalf maximum (FWHM).

In addition, referring to the TEM image of FIG. 3 , the prepared quantumdots have a multi-pod shape. In addition, the quantum dots have aparticle diameter of about 13 nm±1.2 nm (9%).

Inductively coupled plasma atomic emission spectroscopy of the ZnTeSecore and the ZnTeSe (core)/ZnSeS/ZnS (shell) is performed, and theresults listed as aa mole ratio relative to Zn are shown in Table 3.

Example 4: Preparation of ZnTeSe Core/ZnSe/ZnS Shell Quantum Dot

0.5 mole of ZnF₂ and 0.09 mole of ZnCl₂ are added to 100 mL of squalanein a flask at room temperature, and vacuum-treated at 120° C. for 10minutes. The inside of the flask is filled with nitrogen (N₂), and thereaction temperature is raised to 180° C. Subsequently, the ZnTeSe coreprepared in Synthesis Example 1 (reaction time: 60 minutes) is added tothe flask within 10 seconds, 0.5 mmole of Se/TOP is subsequently slowlyadded to the flask, and the reaction temperature is raised to 280° C.Then, 0.7 mmole of S/TOP is added, and the reaction temperature israised to 340° C. and reacted for 10 minutes. Subsequently, a mixedsolution of 0.5 mmole of Se/TOP and 0.9 mmole of S/TOP is slowly added,and reacted for 20 minutes. Lastly, a mixed solution of 0.5 mmole ofSe/TOP and 1.2 mmole of S/TOP is slowly added and reacted for 20minutes.

After the reaction is completed, the reactor is cooled, and the preparednanocrystals are centrifuged with ethanol and dispersed in toluene.

UV-Vis spectroscopy, a photoluminescence analysis, and a transmissionelectron microscopic analysis of the nanocrystal (core/shell) areperformed, and the results are shown in FIG. 4 and Table 3.

Referring to the photoluminescence analysis result, the quantum dots mayexhibit a maximum emission peak of 452 nm (a full width at half maximum(FWHM): 17 nm) and quantum efficiency of 80%. In other words, thequantum dots emit blue light, and have excellent light emittingcharacteristics of high quantum efficiency and a narrow full width athalf maximum (FWHM) are exhibited.

Referring to the TEM image of FIG. 4 , the quantum dots have a cubicshape. In addition, the quantum dots have a particle diameter rangingfrom about 13.5 nm±1.0 nm (7%).

Inductively coupled plasma atomic emission spectroscopy of the ZnTeSecore and the ZnTeSe (core)/ZnSeS/ZnS (shell) is performed, and theresults are listed as a mole ratio relative to Zn in Table 3.

Example 5: Preparation of ZnTeSe Core/ZnSe/ZnS Shell Quantum Dot

0.5 mole of ZnF₂ is added to 100 mL of squalene in a flask at roomtemperature, and vacuum-treated at 120° C. for 10 minutes. The inside ofthe flask is filled with nitrogen (N₂), and the reaction temperature israised to 180° C. Subsequently, the ZnTeSe core prepared in SynthesisExample 2-2 (reaction time: 60 minutes) is added to the flask within 10seconds, 0.5 mmole of Se/TOP is subsequently slowly added to the flask,and the reaction temperature is raised to 280° C. Then, 0.7 mmole ofS/TOP is added, and the reaction temperature is raised to 350° C. for 10minutes. Subsequently, a mixed solution of 0.5 mmole of Se/TOP and 0.9mmole of S/TOP is slowly added, and reacted for 20 minutes. Lastly, amixed solution of 0.5 mmole of Se/TOP and 1.2 mmole of S/TOP is slowlyadded, and reacted for 20 minutes.

After the reaction is completed, the reactor is cooled, and the preparednanocrystals are centrifuged with ethanol and dispersed in toluene.

UV-vis spectroscopy, a photoluminescence analysis, and a transmissionelectron microscopic analysis of the nanocrystal (core/shell) areperformed, and the results are shown in FIG. 5 and Table 3.

Referring to the photoluminescence analysis result, the quantum dotsexhibit a maximum emission peak of 453 nm (a full width at half maximum(FWHM): 18 nm) and quantum efficiency of 76%. In other words, thequantum dots emit blue light, and excellent light emittingcharacteristics of high quantum efficiency and a narrow full width athalf maximum (FWHM) are shown.

Referring to the TEM image of FIG. 5 , the quantum dots mostly have acubic shape. In addition, the quantum dots have a particle diameterranging from about 13.3 nm±1.3 nm (10%).

Inductively coupled plasma atomic emission spectroscopy of the ZnTeSecore and the ZnTeSe (core)/ZnSeS/ZnS (shell) is performed, and theresults are listed as a mole ratio relative to Zn in Table 3.

TABLE 3 A. QY (H (Se + Rxn. PL Co., S)/ temp. Core (@372) Ltd.) Zn Se SZn S/Se Ex. 3 320° C. Syn. 451 21 82 4.05 1.5 2.8 1.06 1.87 Ex. 1 Ex. 4340° C. 452 17 80 4.05 1.5 2.8 1.06 1.87 Ex. 5 350° C. Syn. 453 18 764.05 1.5 2.8 1.06 1.87 Ex. 2-2

Examples 6 and 7: Preparation of ZnTeSe Core/ZnSe/ZnS Shell Quantum Dot

The ZnTeSe core/ZnSe/ZnS shell quantum dots are prepared according tothe same method as Example 3 except that when each of the Se-TOP andS-TOP is added to the reaction flask three separate times to form ashell, the total reaction time is divided into five time intervals, andthe reaction temperature is maintained at 340° C. from the first timeinterval to the third time interval, and at 320° C. (Example 6) or 340°C. (Example 7) in the last two time intervals. UV-Vis spectroscopy and aphotoluminescence analysis of the prepared ZnTeSe core/ZnSe/ZnS shellquantum dots are performed, and the results are shown in Table 4.

Referring to the photoluminescence analysis result, the semiconductornanocrystal particles according to Example 6 exhibit a maximum emissionpeak of 452 nm (a full width at half maximum (FWHM) of 16 nm) andquantum efficiency of 86%. The quantum dots according to Example 7exhibit a maximum emission peak of 453 nm (a full width at half maximum(FWHM) of 17 nm) and quantum efficiency of 81%. In other words, thequantum dots of Examples 6 and 7 emit blue light, and exhibit excellentlight emitting characteristics of high quantum efficiency and a narrowfull width at half maximum (FWHM). In addition, inductively coupledplasma atomic emission spectroscopy of the ZnTeSe core and the ZnTeSe(core)/ZnSeS/ZnS (shell) is performed, and the results (a mole ratiorelative to Zn) are shown in Table 4.

In addition, PL spectra of the quantum dots according to Examples 6 and7 are shown in FIG. 6 , and UV-Vis spectra thereof are shown in FIG. 7 .

TABLE 4 A. QY (H (Se + S)/ Core PL (@372) Co., Ltd.) Zn Se S Zn S/Se Ex.6 Syn. 452 16 86 4.05 1.5 2.8 1.06 1.87 Ex. 1 Ex. 7 453 17 81 4.05 1.52.8 1.06 1.87

Comparative Example 1: Preparation of ZnTeSe Core/ZnS Shell Quantum Dotwithout ZnF₂

0.6 mmole (0.336 g) of zinc acetate, 3.6 mmole (1.134 g) of oleic acid,and 10 mL of trioctylamine are added to a flask and the reaction mixtureis heated under vacuum at 120° C. for 10 minutes. The inside of theflask is filed with nitrogen (N₂), and the reaction temperature israised to 180° C. Then, the ZnTeSe core prepared in Synthesis Example 1(reaction time: 60 minutes) is added to the reaction flask within 10seconds, 0.9 mmole of Se/TOP and 0.9 mmole of S/TOP are subsequentlyadded to the flask, and the reaction temperature is raised to 340° C.and reacted for 10 minutes. Subsequently, a mixed solution of zincacetate and Se/TOP and S/TOP is slowly added in a mole ratio shown inTable 5. A mixed solution of zinc acetate and S/TOP alone is added inthe last addition of the total of four additions.

After the reaction is completed, the reactor is cooled, and the preparednanocrystals are centrifuged with ethanol and dispersed in toluene.

UV-Vis spectroscopy, a photoluminescence analysis, and a transmissionelectron microscopic analysis of the nanocrystal (core/shell) areperformed, and the results are shown in FIG. 8 and Table 5.

Referring to the photoluminescence analysis result, the quantum dotsexhibit a maximum emission peak of 451 nm (a full width at half maximum(FWHM): 22 nm) and quantum efficiency of 77%. In other words, a methodof preparing a semiconductor nanocrystal in which a ZnS shell is formedon a ZnTeSe core without adding ZnF₂ shows excellent quantum efficiency,but a full width at half maximum (FWHM) of greater than 20 nm.Accordingly, the semiconductor nanocrystal prepared from the method doesnot produce a light emitting device with high color reproducibility.

In addition, referring to the TEM image of FIG. 8 , the comparativequantum dots primarily have a multi-pod shape, and the shapes are not asuniform (are of irregular shape). In addition, the quantum dots have aparticle diameter ranging from 13.9 nm±1.9 nm (13%).

Inductively coupled plasma atomic emission spectroscopy of the ZnTeSecore and the ZnTeSe (core)/ZnS (shell) is performed, and the results arelisted as a mole ratio relative to Zn in Table 5.

Comparative Example 2: Preparation of ZnTeSe Core/ZnS Shell Quantum Dotwithout ZnF₂

A quantum dot having a ZnTeSe core/ZnS shell according to ComparativeExample 2 is prepared according to the same as Comparative Example 1except that the Se/TOP content added in each addition is changed asprovided in Table 5.

Referring to the TEM image of FIG. 9 , the quantum dots have a littleirregular shape.

In addition, referring to the photoluminescence analysis result, thequantum dots have a maximum emission peak of 450 nm (a full width athalf maximum (FWHM): 27 nm) and quantum efficiency of 60%. In otherwords, the semiconductor nanocrystals prepared by forming a ZnS shell ona ZnTeSe core without adding ZnF₂ exhibit a peak emission wavelengthrange shifted toward a somewhat shorter blue wavelength, a significantlylower quantum efficiency, and a significantly greater full width at halfmaximum (FWHM) of 27 nm, i.e., relatively poor light emittingcharacteristics.

Inductively coupled plasma atomic emission spectroscopy of the ZnTeSecore and the ZnTeSe (core)/ZnS (shell) is performed, and the results arelisted as a mole ratio relative to Zn in Table 5.

Comparative Example 3: Preparation of ZnTeSe Core/ZnS Shell Quantum Dotwithout ZnF₂

A quantum dot having the ZnTeSe core and the ZnSe/ZnS shell according toComparative Example 3 is prepared according to the same method asComparative Examples 1 or 2, except that the ratio of the zinc precursorand the sulfur precursor added in each section is changed as provided inTable 5. Referring to the photoluminescence analysis result, thesemiconductor nanocrystal particles exhibit a maximum emission peak of449 nm (a full width at half maximum (FWHM): 33 nm) and quantumefficiency of 43%, i.e., light emitting characteristics that are notfavorable even compared to those of Comparative Example 2.

In addition, referring to the TEM image of FIG. 10 , the quantum dotshave a particle size of 15.7 nm±1.4 nm (13%), and are of considerablyirregular shape.

A result of performing inductively coupled plasma atomic emissionspectroscopy of the ZnTeSe core and ZnTeSe (core)/ZnS (shell), i.e., amole ratio relative to Zn, is shown in Table 5.

TABLE 5 ZnSe-ZnS-final 80 ml A. QY (Se + S)/ TEM Core 340° C. PL (@372)(B.C.) Zn Se S Zn Size Comp. Syn. S:0.9-0.9-1.0-1.2, 451 22 77 5.67 1.54.0 0.97 13.9 Ex. 1 Ex. 1 Zn: 0.6-0.75-0.75-0.75, Se: 0.9-0.96-0.96 Com.S: 0.9-0.9-1.0-1.0-1.2, 450 27 60 6.11 1.5 5.0 1.06 14.5 Exa. 2 Zn:0.6-0.75-0.75-0.75, Se: 0.8-0.8-0.83-0.83 Com. S: 0.9-0.9-1.0-1.0-1.0-449 33 43 8.00 1.5 7.0 1.06 15.7 Exa. 3 1.0-1.2, Zn: 0.6-0.75-0.75-0.75,Se: 0.8-0.8-0.85-0.9- 0.9-0.9

Comparative Examples 4 and 5: Preparation of ZnTeSe Core/ZnS ShellQuantum Dot Using Other Metal Fluoride Except ZnF₂

A quantum dot having a ZnTeSe core/a ZnSe/ZnS shell is prepared under asimilar condition to that of Example 3, but Comparative Example 4includes 0.35 mole of AlF₃ instead of ZnF₂ at room temperature, andComparative Example 5 includes 1.06 mole of LiF instead of ZnF₂ at roomtemperature.

As a result, Comparative Examples 4 and 5 as shown in Table 6, thequantum dots prepared by including AlF₃ or LiF instead of ZnF₂respectively exhibit a shorter emission peak wavelength of 442 nm and443 nm, respectively, and also, luminous efficiency decreased to 50%. Inother words, when the metal of the metal fluoride is not Zn, theresulting prepared quantum dots exhibit light emitting characteristicsthat are very different than the quantum dots prepared by including ZnF₂or a precursor thereof, zinc-carboxylate, or a fluorine ion source.

The photoluminescence (PL) spectra for the quantum dots of ComparativeExamples 4 and 5 are shown in FIG. 11 , and the UV-Vis spectra thereofare shown in FIG. 12 .

TABLE 6 A. QY ZnSe-ZnS-final 80 ml PL (H Co., (Se + S)/ Core scale, 340°C. (@372) Ltd.) Zn Se S Zn S/Se Comp. Syn. Se: 0.5-0.5-0.5, 442 16 504.05 1.5 2.8 1.06 1.87 Ex. 4 Ex. 1 S: 0.7-0.9-1.2, Zn:0.6-0.75-0.75-0.75- 0.6-0.6, addition 0.35 of AlF₃ at room temperatureComp. Se: 0.5-0.5-0.5, 443 17 52 4.05 1.5 2.8 1.06 1.87 Ex. 5 S:0.7-0.9-1.2, Zn: 0.6-0.75-0.75-0.75- 0.6-0.6, addition 1.06 of LiF atroom temperature

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A quantum dot comprising a core comprising afirst semiconductor nanocrystal comprising zinc and selenium, andoptionally sulfur and/or tellurium, and a shell comprising a secondsemiconductor nanocrystal comprising zinc and at least one of sulfur orselenium, wherein the quantum dot has an average particle diameter ofabout 13 nanometers to about 20 nanometers, and an emission peakwavelength in a range of about 440 nanometers to about 470 nanometers,and a full width at half maximum (FWHM) of an emission wavelength ofless than about 25 nanometers, wherein the quantum dot exhibits aquantum efficiency of greater than to equal to about 70%, and whereinthe quantum dot is cadmium-free.
 2. The quantum dot of claim 1, whereinthe quantum dot has an emission peak wavelength in a range of about 445nanometers to about 460 nanometers.
 3. The quantum dot of claim 1,wherein the quantum dot exhibits a quantum efficiency of greater than orequal to about 80%.
 4. The quantum dot of claim 1, wherein the firstsemiconductor nanocrystal comprises zinc and selenium, or zinc,selenium, and tellurium.
 5. The quantum dot of claim 1, wherein thesecond semiconductor nanocrystal comprises zinc and selenium, or zincand sulfur.
 6. The quantum dot of claim 1, wherein the secondsemiconductor nanocrystal comprises zinc, selenium, and sulfur.
 7. Thequantum dot of claim 1, wherein a ratio of total moles of sulfur andselenium relative to the moles of zinc in the second semiconductornanocrystal is about 1 to about
 2. 8. A quantum dot-polymer compositecomprising a polymer matrix; and the quantum dot of claim 1 dispersed inthe polymer matrix.
 9. An electronic device comprising the quantum dotof claim 1.